1. Field of the Invention
This invention relates generally to a pump configured to transfer blood at a relatively low pressure and high volume, and, more particularly, to a pump configured to pump human blood while avoiding damage to the components of the blood, such as red corpuscles and platelets.
2. Description of Related Art
There have been various attempts to design a pump to replace the functions of the human heart. The attempts to date have been marred by a considerable number of problems, due in large part to the complicated nature of the human heart, as well as the tendency of blood components to be damaged when they come in contact with mechanical components under certain conditions of shear or stress.
While the most obvious replacement for a human heart would involve an emulation of the mechanical features of the human heart, previous attempts at such a solution have met with problems. The relatively large number of moving parts and reciprocating action of artificial hearts, such as the JARVIK heart, have contributed to mechanical complexity and unreliability and the configurations have made the creation of a small replaceable artificial heart difficult.
One alternative approach to direct emulation of the human heart has been to propose rotational pumps in order to gain the benefits of small size and mechanical simplicity. However, such pumps have demonstrated a tendency to damage the blood, resulting in fibrin accumulation and hemolysis, which makes the pumps unsuitable, particularly if a rotational pump is to be implanted as an artifical hart for long term use.
One approach chosen in the past has been to model the heart by designing a pump that mimics the kinematics of the heart, which is similar to the kinematics of a positive displacement pump. In a positive displacement pump, fluid is moved in discrete quantities, necessitating the use of some form of check valve to separate the quantities of fluid as they are moved.
While mechanical positive displacement pumps can mimic the human heart, they are by no means the ideal mechanical heart. Positive displacement pumps are typically bulky, complicated, and require artificial valves. It is also difficult to recreate the heart's reciprocating motion. In addition, there are often many hidden recesses in positive displacement pumps where blood can stagnate. Blood damage due to stagnation and turbulence also typically occurs during the beginning and the end of the strokes of fixed displacement blood pumps, as the piston or diaphragm reverses direction. Centrifugal pumps have also been designed to pump blood, and have inherent advantages over positive displacement blood pumps, but in practice have also shown a tendency to damage the blood during prolonged use.
The human heart also maintains a variable output pressure and flow schedule in the presence of a varying resistance caused by vasodilation and vasocontraction of the arteries as a result of changing demand for oxygen. In practice, this requires that a pump designed to mimic the heart have some form of feedback to control the speed of the pump and regulate the volume of fluid being pumped, so that a change in the back pressure results in a change in the speed of the pump. In the human heart, this speed control is automatically governed by the central nervous system in the form of changes to the heartbeat.
Another important factor in designing an artificial heart to pump blood is the potential for destruction of blood components when blood comes in contact with mechanical surfaces. Specifically, it is known that damage to blood components such as erythrocytes (red blood cells) and platelets in a mechanical pump is a function of the stress on the blood flow stream and the time the blood is exposed to the inside of the pump. These two factors are herein referred to as the "stress-time product." Each factor in turn depends on a number of variables, and those factors form design parameters in the design of a blood pump. Typically, stress imparted to a fluid flow stream is a function of the velocity of the surface in the pump that is driving the fluid, be it a rotor vane, piston face, or the walls of a contracting chamber. The faster the velocity of the pump driving surface, the greater the stress imparted to the fluid, and the greater the potential for surface-induced damage to the blood components. In conventional pumps, in order to slow down the velocity of the pump driving surface, without changing the amount of volume-flow that can be pumped by the pump, it is necessary to increase the area of the pump driving surface. But increasing the area also increases the size of the pump, which is a drawback to be avoided, especially if the pump is to be implanted.
The time and character of exposure of blood inside a pump as exemplified by a stress-time parameter are also critical issues in designing a heart pump. Excessive exposure time of blood to stress such as shear inside the pump can cause coagulation, emboli and fibrin accumulation, the latter manifesting itself in string-like particulate matter forming on the pump surfaces. In addition, flow separation, cavitation and swirl of blood streamlines can produce undesirable thrombus material.
The flow in a centrifugal pump, unlike the flow in a positive-displacement pump, can be continuous and non-pulsating. This results in a lower maximum velocity, and consequently the stress imparted by the pump surfaces on the blood is lower. Tests have shown that the maximum velocity in a centrifugal-type blood pump can be significantly less than the maximum velocity in a positive displacement-type blood pump. Animal tests have also shown acceptable longtime performance with steady state flow. However, some researchers feel that certain organs such as the kidneys may be affected by nonpulsing flow.
Thus, there remains a need for a compact, reliable, and simple blood pump that may be used to temporarily or permanently replace a defective human heart. It would be advantageous if such a blood pump was easy to manufacture, placed relatively little stress upon the blood components being pumped, and was easily driven by an external power source. The present invention satisfies all of these requirements.